Fiber-optic communications is a most important method of communications. A large amount of information is transferred via fiber optics systems. Examples of such communications beyond telephones are Internet and cable TV networks. The present commercial transmission rate of a single fiber with a single wavelength can reach as high as 10 Gb/second. A way to expand the transmission capacity is by the use of more than one wavelength on a single optical fiber. This technology is called WDM.
The state of the art in dense WDM (DWDM) systems allow hundreds of channels per fiber, and this capacity is expected to increase. Most of today's communication systems are known as passive systems, meaning that the particular wavelengths are assigned to particular addresses and there is a limited ability to change these dynamically during the communication. Dynamically tunable light sources that might be used for routing of optical information packets exist, however, today they are not well developed. For this purpose and others, it is needed to have a discrete grid of wavelengths that should be known to all nodes in the network. By using a plurality of wavelengths, it is possible to build many virtual connection paths between different nodes that are connected with each other using a small number of physical links. The separation between different channels is done using optical filtering (or wavelength de-multiplexing). Therefore, for communication uses, it is preferable that tunable laser sources are tuned within a stable discrete grid of wavelengths that are used in the optical network. There are also other needs for a tunable laser, such as the simple use as a spare to replace losers with any wavelength in the WDM system that stops functioning. A fast tunable laser is known in the art, as disclosed by Vljaysekhar Jayarman et al, IEEE JQE 29, p.92 (1993).
A fast tunable, mode-locked laser, as disclosed in U.S. Pat. No. 6,389,047 to Fischer, inventor of the present invention, is operated at particularly chosen wavelengths with the ability to switch from wavelength to wavelength. This patent provides for a collection of reflectors, which gradually increase or decrease in their central reflective wavelength. It allows selection and high speed switching between these wavelengths. This can be useful for dynamic wavelength division multiplexing (WDM) in which fiber optic communication systems having multiple wavelength channels in a single fiber, each channel carrying information which can be routed and processed independently. In one particular realization of the laser in this patent, the semiconductor gain element was high reflection coated at one end, thus producing a broadband reflecting mirror, and anti-reflection coated (ARC) at the other end, from which the cavity is extended into a complex fiber Bragg grating (FBG) mirror, consisting of a plurality of selective (in wavelength) reflectors (mirrors), one after the other. These reflectors define different lengths of the cavity. As a result, every cavity length has a different mode-locking frequency. Different cavity lengths, through their selective mirrors, in turn, define different wavelengths of the laser operation according to the associated selective mirror or fiber Bragg grating section that forms the cavity. Selecting the lasers operation wavelength is done by choosing and applying the mode locking frequency that matches the length of this cavity.
The prior art has several drawbacks that need to be addressed. To achieve accurate wavelengths, the spectral bandwidth of the Bragg reflectors has to be narrow, thus the overall plurality of reflectors has to be quite long. That is, for the laser to support a large number of wavelengths, a relatively long multiple element Bragg reflector is required, consisting of many different reflectors positioned one after another. This causes large discrepancies between the roundtrip time of the shortest cavity formed with the reflector that is the closest to the laser facet, and the longest one formed with the last Bragg reflector. This therefore complicates the cavity design from the degeneracy of harmonics mode-locking point of views and increases the differences between the bit-rates of the different emitted wavelengths. To reduce the complexity of designing transceivers, it is preferred to minimize the Bragg reflector length.
In order to relieve the above drawbacks, the length of each reflector should be short and consecutive reflectors should be closely spaced. This process eventually creates a chirped grating, assuming a monotonic increase or decrease of wavelengths from beginning to end of the reflector. Theoretically, when a chirped grating is used (instead of a discrete reflector for each wavelength), the source can emit any wavelength from the band of wavelengths reflected by the chirped grating. This should be done by continuously tuning of the driving modulation frequency. Due to the fact that the mode-locking process has a finite bandwidth, a given modulation frequency, fRf, (nominally equals (c/n)/(2L0) or any integer multiples of it, where c/n is the effective speed of light in the cavity and L0 is a nominal cavity length) can produce mode locking for cavity whose length L0 is not exactly matched; all the cavities of length in the range L0±ΔL will almost equally support mode-locking for the same driving frequency (the higher the cavity Q factor, the lower ΔL). This fact raises wavelength stability problems for the extended cavity with the chirped grating; all the wavelengths that are found in the strip of width ΔL in the chirped grating are allowed. Thus, for a given driving modulation frequency, there is uncertainty in the actual emitted wavelength. Therefore, the possible continuous wavelength selection with a chirped grating reflector (which can be advantageous for some uses) needs some extra consideration and means to tune and stabilize the wavelength accurately to the precise needs of DWDM standards.
Therefore, it would be desirable to provide a system and method for a high-speed wavelength selection providing stabilized operation over a set of discrete wavelengths in a single laser source, to enable use of said system in dynamic wavelength division multiplexing communication methods and other systems.